Al-plated steel sheet for exhaust gas passageway members of motorcycles excellent in high-temperature strength and the members

ABSTRACT

Provided is an Al-plated steel sheet for motorcycle exhaust gas passageway members excellent in high-temperature strength and red scale resistance, which is produced by dipping a substrate steel sheet comprising, in terms of % by mass, at most 0.02% of C, at most 2% of Si, at most 2% of Mn, from 5 to 25% of Cr, from more than 0.1 to 1% of Nb, at most 0.3% of Ti, at most 0.02% of N, and optionally at least one of at most 0.6% of Ni, at most 0.2% of Al, at most 3% of Mo, at most 3% of Cu, at most 3% of W, at most 0.5% of V, at most 0.5% of Co and at most 0.01% of B, with a balance of Fe and inevitable impurities, in a hot-dip Al-base plating bath to thereby form a hot-dip plating layer having a mean thickness of from 3 to 20 μm on the surface thereof.

TECHNICAL FIELD

The present invention relates to an aluminium-plated steel sheetexcellent in high-temperature strength and red scale resistance for usein motorcycle engine exhaust gas passageway members (e.g., mufflers,catalyst carriers, exhaust pipes, etc.), and to a motorcycle exhaust gaspassageway member comprising it.

PRIOR ART

The engine exhaust gas passageway in motorcycles is shorter than that infour-wheel vehicles, and not unusually, even the downstream members suchas mufflers and others may be often exposed to high temperatures notlower than 400° C. With recent reinforcement for exhaust gas regulation,an exhaust gas purification device comprising a ternary catalyst isbeing built also in motorcycles; and in motorcycles, the purificationdevice is often incorporated in the muffler therein. Heat-resistantferritic stainless steel such as SUH409L, SUS436 or the like is used formotorcycle exhaust gas passageway members from the viewpoint of heatresistance.

Exhaust gas passageway members comprising stainless steel may often havered-brown scale (hereinafter referred to as “red scale”). This is aphenomenon often occurring in a low-oxygen high-moisture atmosphere atan exhaust gas temperature of from 400 to 700° C. When red scale hasformed and when it is mixed with dew condensation water, then it gives ared-brown liquid. The dew condensation water in an exhaust gaspassageway may be discharged out through an exhaust port along withexhaust gas therethrough, but it is extremely unfavorable to dischargethe above-mentioned red-brown liquid in the appearance.

Of exhaust gas passageway members, a muffler is a site where dewcondensation water may readily remain. In ordinary four-wheel vehicles,the muffler is rarely exposed to exhaust gas at 400° C. or higher, andthe corrosion caused by internal dew condensation water is oftenproblematic rather than the red scale to form inside the muffler.However, in motorcycles, even the muffler may be frequently exposed to400° C. or higher as so mentioned in the above, and therefore red scaleforms inside the muffler and the release of a red-brown liquid caused byit is often problematic.

Patent Reference 1 discloses a technique of inhibiting red scaleformation in stove combustion cylinders and others by previously forminga Cr oxide-base oxide film on the surface of the structure. However, theoxide film is poor in corrosion resistance and therefore requires acountermeasure to enhance the corrosion resistance of the steel base,which brings about the increase in the material cost. Accordingly, thetechnique is difficult to apply to exhaust gas passageway members.Patent References 2 and 3 disclose a technique of inhibiting red scaleformation by the use of steel with much Al and Si added thereto. Theshapability of such a high-Al, Si steel may be good as compared withthat of SUH21 (18Cr-3Al steel of good scale resistance), but is muchinferior to that of ferritic stainless steels such as SUH409L, SUS410L,SUS430LX and the like. Therefore, the technique is unsuitable tomotorcycle exhaust gas passageway members.

On the other hand, Al-plated steel sheets of Cr-containing steel such asstainless steel or the like exhibit good oxidation resistance, andvarious types of such steel sheets have been developed for automobileexhaust system members (Patent References 4 to 13).

Patent Reference 1: JP-A 2001-240911

Patent Reference 2: JP-A 2001-316773

Patent Reference 3: JP-A 2003-160844

Patent Reference 4: JP-A 8-319543

Patent Reference 5: JP-A 5-112859

Patent Reference 6: JP-A 5-295513

Patent Reference 7: JP-A 61-147866

Patent Reference 8: JP-A 61-147865

Patent Reference 9: JP-A 7-233451

Patent Reference 10: JP-A 63-47356

Patent Reference 11: JP-A 3-277761

Patent Reference 12: JP-A 7-188887

Patent Reference 13: JP-A 8-325691

PROBLEMS THAT THE INVENTION IS TO SOLVE

According to the present inventors' investigations, Al-plating iseffective for enhancing the resistance to red scale formation(hereinafter referred to as “red scale resistance”) of a Cr-containingsteel sheet such as stainless steel or the like. However, the presentinventors' further detailed investigations on the above-mentioned knownAl-plated steel sheets have revealed that these have some problems to besolved in realizing their broad-range practicable application tomotorcycle exhaust gas passageway members.

Specifically, in the Al-plated steel sheet in Patent Reference 4, Mn andthe like is thickened in the alloy layer, and the plating layerthickness could not be said to be sufficiently thin, and therefore, itis recognized that the peeling resistance of the plating layer (in thisdescription, a hot-dip plating layer including an alloy layer isreferred to as “plating layer”) is insufficient. In the plated steelsheets in Patent References 5 and 6, the plating layer is thick, andtherefore also in these, the peeling resistance of the plating layer isinsufficient. The plated steel sheets in Patent References 7, 8 and 10are produced by Al plating after Ni pre-plating; however, the Nipre-plating employed therein much increases the production costs andtherefore could not be directly applied to exhaust gas passagewaymembers such as mufflers, catalyst carriers and the like for which costreduction is much desired. In these, in addition, the peeling resistanceof the plating layer when heated up to a range of from 400 to 700° C. isnot always on a satisfactory level. In the plated steel sheet in PatentReference 9, the substrate steel indispensably contains a rare earthelement or Y added thereto, and is therefore protected from abnormaloxidation at a high temperature of from 1150 to 1250° C.; however, sincethe adhesiveness between the plating layer and the substrates isinsufficient, the plating layer tends to readily peel away when theplated steel sheet is exposed to cycles of heating to a range of from400 to 700° C. followed by cooling. In the plated steel sheets in PatentReferences 11 and 12, the plating layer is not sufficiently thin, andtherefore the peeling resistance of the plating layer to cycles ofheating to a range of from 400 to 700° C. followed by cooling isinsufficient. “Red rust” described in Patent Reference 11 is typical redrust generally seen in ordinary steel that has been much corroded atroom temperature, and this differs from “red scale” as referred toherein.

As in the above, basically in known Al-plated stainless steel sheets,the peeling resistance of the plating layer could not be said to besatisfactory when heated in a temperature range of from 400 to 700° C.Accordingly, when conventional Al-plated stainless steel sheets areapplied to mufflers, catalyst carriers and the like in motorcycles thatare used in a temperature range of from 400 to 700° C., they couldexhibit good corrosion resistance and red scale resistance in the earlydays; however, while used for a long period of time, the plating layermay peel away, therefore causing reduction in the corrosion resistanceand reduction in the red scale resistance of the steel sheets. In otherwords, they involve some risk factors in point of the durabilitythereof. On the other hand, various types of motorcycles with an exhaustgas purification catalyst built therein are increasing owing to therecent tendency toward reinforcement for exhaust gas regulation. Whensuch a catalyst is built in motorcycles, the temperature of the exhaustgas from them becomes higher due to the reaction and the exhaust gaspassageway members such as mufflers and others therein shall be exposedto further high temperatures. Accordingly, it has become desired toapply materials having much better high-temperature strength than beforeto exhaust gas passageway members of motorcycles. In addition, the steelsheets for exhaust gas members are required to have good shapability andlow-temperature toughness. Further, low-cost production is an importantfactor for industrial applicability.

An object of the present invention is to provide an Al-plated steelsheet for motorcycle exhaust gas passageway members, which isinexpensive and has good high-temperature strength and is excellent inred scale resistance, shapability and low-temperature toughness and inwhich the peeling resistance of the plating layer in repeated heating ina temperature range of from 400 to 700° C. has been significantlyenhanced.

MEANS FOR SOLVING THE PROBLEMS

As a result of detailed investigations, the present inventors have foundthat, when the thickness of the Al-base plating layer is controlled tobe at most 20 μm, then the peeling resistance of the Al-base platinglayer can be significantly enhanced, and have completed the presentinvention.

Specifically, the invention provides an Al-plated steel sheet formotorcycle exhaust gas passageway members excellent in red scaleresistance, which is produced by dipping a substrate steel sheet havinga base steel composition comprising, in terms of % by mass, at most0.02% of C, at most 2% of Si, at most 2% of Mn, from 5 to 25% of Cr,from more than 0.1 to 1% of Nb, at most 0.3% of Ti, at most 0.02% of N,and optionally at least one of at most 0.6% of Ni, at most 0.2% of Al,at most 3% of Mo, at most 3% of Cu, at most 3% of W, at most 0.5% of V,at most 0.5% of Co, at most 0.01% of B, with a balance of Fe andinevitable impurities, in a hot-dip plating bath containing, in terms of% by mass, from 3 to 12% of Si and optionally at least one of Ti, B, Sr,Cr, Mn, Mg and Zr in a total amount of at most 1%, with a balance of Aland inevitable impurities, then pulling it up, and controlling theplating amount to thereby form a plating layer having a mean thicknessof from 3 to 20 μm on the surface thereof.

The invention also provides a motorcycle exhaust gas passageway memberwhich is formed of the above-mentioned plated steel sheet and which isso designed that the above-mentioned plating layer thereof is kept incontact with exhaust gas and that the maximum service temperature is400° C. or higher.

The Al-plated steel sheet of the invention is excellent in red scaleresistance and is excellent in peeling resistance of the plating layerin repeated heating in a temperature range of from 400 to 700° C., andtherefore, it is favorable for motorcycle exhaust gas passageway members(e.g., muffler members) that are used in an environment where red scalereadily forms in heating in that temperature range. Since a substratesteel sheet having good high-temperature strength is used, the latitudein planning the exhaust gas passageway member is broadened, and this isespecially advantageous in some types of motorcycles which are equippedwith an exhaust gas purification catalyst built therein and in which thetemperature of the exhaust gas may be therefore high. In addition, theproduction costs may be reduced to at most the same level as that forordinary Al-plated stainless steel sheets. Accordingly, the inventionmay contribute toward improving and enhancing the quality and thedurability of motorcycle exhaust gas passageway members.

PREFERRED EMBODIMENTS OF THE INVENTION

When a high-Cr steel such as ferritic stainless steel or the like isheated at a high temperature, in general, an oxide having a high Crconcentration (Cr-base oxide) is formed in the surface of the steelbase. Since the Cr-base oxide has a high protecting capability, thehigh-Cr steel may generally have good high-temperature oxidationresistance as compared with low-Cr steel. However, it is known that,when heated in a low-oxygen high-moisture atmosphere in a temperaturerange of from 400 to 700° C., even such a high-Cr steel readily forms anFe-base oxide rather than a Cr-base oxide in the initial stage of theoxidation process thereof. This is considered because Cr could notrapidly diffuse in the surface of the steel base in an amount enough tocover the surface in the form of the Cr-base oxide thereof but rather Feexisting in a large amount in the surface would be predominantlyoxidized in the low-oxygen high-moisture atmosphere. Red scale iscomposed of the oxide having a high Fe concentration formed in thesurface in the manner as above.

For preventing the red scale formation, the Fe-base oxide may beprevented from forming in the surface of the steel base in theatmosphere and the temperature range mentioned above. As one means forit, Al-base plating may be effective. In this case, Al in the surface ofthe plating layer may be rapidly oxidized, and the surface of the steelsheet may be covered with the Al-base oxide film.

However, the present inventors' investigations have revealed that theAl-base plating layer may readily peel off from the surface of the steelbase in repeated cycles of heating up to a temperature of from 400 to700° C. followed by cooling. This is a significant risk factor of notalways exhibiting good durability in application of conventionalAl-plated steel sheets to motorcycle muffler members or the like. When ahot-dip Al-plated steel sheet is heated at 400 to 700° C., Al in theplating layer and Fe in the steel base interdiffuse to give an Fe—Alintermetallic compound layer, and the intermetallic compound layer peelsoff from the steel base. In that manner, the plating layer peels offfrom the steel base, from which red scale begins to newly form.

The present inventors have made detailed studies about the method ofpreventing the plating layer from peeling off from the Al-plated steelsheet. As a result, the inventors have found that reducing the thicknessof the Al-base plating layer is extremely effective for preventing theAl-base plating layer from peeling off from the steel base in heating ina low-oxygen high-moisture atmosphere in a temperature range of from 400to 700° C. In this case, there is no necessity of providing any specificlimitation on the chemical composition of the base steel sheet, on thecomposition and the texture condition of the Al-base plating layer andon the composition of the alloy layer.

[Thickness of Al-Base Plating Layer]

Concretely, in the hot-dip Al-plated steel sheet, the mean thickness ofthe Al-base plating layer is defined to be at most 20 μm per onesurface, whereby the plating layer can exhibit excellent peelingresistance. The mean thickness of the Al-base plating layer is thethickness including no alloy layer. As well known, the hot-dip platingamount can be controlled according to a gas wiping method or the like,and therefore, in case where the plating amount itself per one surfaceis controlled to be at most 20 μm, then a part thereof could react withthe steel base to form an alloy layer, and the mean thickness of theformed Al-base plating layer is not more than 20 μm. Under the operationcondition under which a relatively thick alloy layer could be formed,the mean thickness of the plating layer could be at most 20 μm as thecase may be, even when the plating amount is set to be somewhat largerthan 20 μm. When the mean thickness of the Al-base plating layer is lessthan 15 μm, then the peeling resistance may better further more.Accordingly, a case where the mean thickness of the Al-base platinglayer per one surface is less than 15 μm is an especially preferredembodiment of the invention.

On the other hand, from the viewpoint of preventing red scale, thethickness of the Al-base plating layer is preferably larger. As a resultof various investigations, the mean thickness of the Al-base platinglayer per one surface must beat least 3 μm on the precondition of usinga base steel sheet having a controlled composition as described below.When the plating layer is thinner than the range, it may be difficult tostably prevent the red scale formation. More preferably, the meanthickness is at least 4 μm, even more preferably at least 5 μm.

[Thickness of Alloy Layer]

The alloy layer is relatively more brittle as compared with the steelbase and the plating layer. Accordingly, the thickness of the alloylayer is preferably thinner in consideration of the workability of theplated steel sheet. Any specific consideration may be unnecessary foruse of the plated steel for low working ratio members; however, when theplated steel sheet is severely deformed, then the mean thickness of thealloy layer per one surface is preferably at most 5 μm, more preferablyat most 4 μm. In a hot-dip Al-base plating line using a gas-wipingmethod, it is well possible to produce plated steel sheets having such athin alloy layer. Regarding the total thickness of the plating layer andthe alloy layer, the thickness of the alloy layer is preferably socontrolled that the “mean thickness of the Al-base plating layer+meanthickness of the alloy layer” per one surface could be at most 24 μm.

[Composition of Al-Base Plating Bath]

In an Al—Si alloy system, the liquidus-line temperature lowers withaddition of Si to Al, and the system may have an eutectic compositionwhen the Si content thereof reaches about 12% by mass. In the invention,a hot-dip Al-base plating bath containing Si in an amount of at least 3%by mass is used. An Al-richer composition than those of the bath willneed a higher bath temperature, and in such a plating bath, it may bedifficult to control the mean thickness of the alloy layer to be thin(for example, at most 5 μm). In addition, the increase in the bathtemperature may cause the increase in the production cost. However, whenthe Si content is more than 12% by mass, then the workability of theAl-base plating layer itself may worsen, therefore often bringing abouta problem. Accordingly, in the invention, a hot-dip Al-base plating baththat contains from 3 to 12% by mass of Si is used to produce theintended plated steel sheet.

The Al-base plating bath may contain at least one of Ti, B, Sr, Cr, Mn,Mg and Zr in a total amount of at most 1%. In addition, the bath maycontain Fe as an inevitable impurity, in which Fe is allowable within arange of at most 2.5% by mass.

[Substrate Steel Sheet]

The substrate steel sheet to be plated is a high-Cr steel sheetcontaining from 5 to 25% by mass of Cr. Since the corrosion resistanceand the oxidation resistance of the steel sheet could be enhanced byAl-plating, the substrate steel sheet is not always required to have aCr content on a level of stainless steel; however, in order that theplated steel sheet could secure the necessary corrosion resistance andred scale resistance for motorcycle exhaust gas passageway members thatare exposed to an environment where they are kept in contact with dewcondensation water and water vapor therein, the substrate steel sheetmust have a Cr content of at least 5% by mass. More preferably, the Crcontent is at least 10% by mass. With the increase in the Cr content,the corrosion resistance and the heat resistance of the steel sheet maybetter; however, too much Cr existing in the steel sheet is uneconomicaland may cause a factor of interfering the shapability and thelow-temperature toughness of steel. Accordingly, the Cr content of thesubstrate steel sheet is defined to fall within a range of at most 25%by mass.

Nb has an effect of enhancing the high-temperature strength of steel. Incase where steel contains relatively much C and N, then Nb therein mayhave an effect of enhancing the low-temperature toughness of the steel.In order to fully attain these effects, in the invention, a ferriticsteel having an Nb content of more than 0.1% by mass is employed.However, the present inventors' studies have revealed that Nb in asubstrate steel sheet may detract from the peeling resistance of thehot-dip Al-base plating layer. As a result of detailed investigations,the inventors have found that the Nb content of the substrate steelsheet is allowable to a level of up to 1% on the precondition ofcontrolling the mean thickness of the Al-base plating layer to be atmost 20 μm as so mentioned in the above. Accordingly, in the invention,the substrate steel sheet to be used contains Nb in an amount fallingwithin a range of from more than 0.1 to 1%.

Ti is an element effective for fixing C and N in steel and forstabilizing the ferrite phase and further for enhancing thelow-temperature toughness and the shapability of steel. In order to makethe element fully exhibit these effects thereof, preferably the Ticontent is at least 0.05% by mass, more preferably at least 0.1% bymass. However, too much Ti may harden steel and therefore may ratherworsen the workability and the low-temperature toughness of steel.Accordingly, the Ti content is limited to fall within a range of at most0.3% by mass, and more preferably, it is within a range of at most 0.2%by mass.

C, Si, Mn and N are basic elements in steel. C may be in an amountfalling within a range of at most 0.02% by mass, and N may be in anamount of at most 0.02% by mass. When the content of Si and Mnincreases, then the low-temperature toughness of steel may lower; but inthe invention, both Si and Mn are allowable within a range of up to 2%by mass each. As other elements, the substrate steel may contain atleast one of at most 0.6% of Ni, at most 0.2% of Al, at most 3% of Mo,at most 3% of Cu, at most 3% of W, at most 0.5% of V, at most 0.5% of Coand at most 0.01% of B; however, when the content of these elements ismore than the above-mentioned limitation, then they may have somenegative influences on the shapability and the low-temperature toughnessof steel. As inevitable impurities, P may be allowable in an amount ofat most 0.10% by mass or so, and S may be in an amount of at most 0.03%by mass or so.

[Production Method]

The substrate steel sheet may be produced according to an ordinary steelsheet production process, and the production method for it is notspecifically defined. For example, pickling-finished cold-rolled steelsheet may be used as the substrate; and while the surface of thesubstrate steel sheet is kept activated, the sheet is dipped in ahot-dip Al-base plating bath and then pulling it up, and the platingamount is controlled to produce the hot-dip Al-plated steel sheet of theinvention. For enhancing the platability thereof, an Fe pre-platedsubstrate steel sheet maybe employed. A strip of the substrate steelsheet is introduced into a continuous hot-dip plating line, and ahot-dip Al-plated steel sheet of high quality can be thereby producedstably in a mode of industrial-scale mass production. Regarding theplating condition, it is important that the line speed and the wipingcondition are so controlled that the mean thickness of the plating layercould be at most 20 μm per one surface; but for the others, ordinaryconditions may be employed. Thus obtained, the plated steel sheet may beprocessed in a predetermined shaping and deforming process to giveexhaust gas passageway members for motorcycles. For some members, thesheet may be welded into pipes and then and deformed shaped.

Examples

Ferritic steels each having the composition shown in Table 1 wereproduced through melting, and then processed according to an ordinarymethod to give cold-rolled annealed steel sheets (pickling-finishedsteel sheets) having a thickness of 1.2 mm. As the inevitable impuritiesin all these steels, P was at most 0.10% by mass and S was at most 0.01%by mass. These steel sheets were used as substrates, and variouslyplated in a mode of hot-dip Al-base plating. In Nos. 11 and 27, thesurface of the cold-rolled annealed steel sheet was pre-plated with Fe(2 g/m²), and these were used as substrates. In all Examples, theplating bath contained an inevitable impurity Fe in an amount of 1.7% bymass or so. The cross section of the obtained, hot-dip Al-plated steelsheet (sample sheet) was observed with SEM (scanning electronicmicroscope), and the mean thickness of the Al-base plating layer wasdetermined. On that occasion, the mean thickness of the alloy layer wasalso determined, and as a result, it was at most 4 μm in all casesexcept some comparative examples.

TABLE 1 Steel Chemical Composition (% by mass) Classification Code C SiMn Ni Cr Nb Ti Al N Mo Cu W V Co B Steel of the X1 0.009 0.46 0.80 0.0812.05 0.51 0.07 0.03 0.012 — — — — — Invention X2 0.006 0.93 0.25 0.1010.94 0.25 0.15 0.02 0.008 — — — 0.03 — 0.003 X3 0.007 0.25 0.36 0.2218.10 0.18 0.19 0.08 0.010 — — — 0.04 — — X4 0.006 0.24 0.49 0.30 24.600.33 0.07 0.09 0.007 — — — 0.03 — — X5 0.010 0.38 1.50 0.34 10.21 0.800.18 0.06 0.009 — — — 0.04 — — X6 0.018 0.60 0.99 — 17.50 0.45 0.20 0.050.011 — — — — — — X7 0.016 1.10 0.28 0.50 18.32 0.60 0.08 0.04 0.009 — —— 0.07 — — X8 0.012 0.44 0.60 — 13.80 0.77 0.15 0.10 0.015 — — — 0.03 —— X9 0.009 0.39 0.55 0.26 12.91 0.39 0.18 — 0.008 1.11 — — 0.04 — — X100.007 0.74 0.31 0.37 14.60 0.33 0.09 0.06 0.007 — 2.10 — 0.05 — — X110.005 0.25 1.15 0.44 19.62 0.47 0.17 0.07 0.009 — — 2.220 0.06 — — X120.011 1.00 0.33 0.21 16.29 0.69 0.06 0.08 0.010 — — — 0.31 — — X13 0.0130.83 0.28 0.19 17.40 0.24 0.05 0.09 0.005 — — — 0.07 0.490 — X14 0.0140.25 1.10 0.15 12.99 0.89 0.11 — 0.009 — — — 0.04 — 0.009 X15 0.010 0.720.65 — 17.42 0.33 0.12 — 0.010 — — — — — — Comparative Y1 0.009 0.600.50 0.26 13.55 1.04 0.29 0.04 0.009 — 0.12 — 0.04 — — Steel Y2 0.0070.80 0.85 0.36 27.30 0.30 0.18 0.04 0.009 0.09 0.11 — 0.05 — — Y3 0.0110.33 0.69 0.19 19.26 0.69 0.38 0.03 0.010 0.07 0.09 — 0.03 — — Y4 0.0090.56 1.25 0.11 18.63 0.05 0.16 0.03 0.012 3.10 0.18 — — — — Y5 0.0080.90 1.09 0.09  4.88 0.59 0.13 0.02 0.008 0.10 — — 0.04 — — Y6 0.0100.48 0.69 0.51 15.39 0.43 0.07 0.06 0.009 0.08 3.55 — 0.05 — —Underlined: falling outside the scope of the invention.

The sample sheets were evaluated for the red scale resistance, thepeeling resistance of the plating layer, the shapability and thelow-temperature toughness in the manner mentioned below.

[Red Scale Resistance]

A test piece of 25 mm×35 mm was cut out of the sample sheet; and thiswas tested in the following two heating modes of oxidation test. Inevery test mode, the number of the test pieces was n=3.

-   -   Continuous heating: In a nitrogen atmosphere having a dew point        of 80° C., the test piece is continuously heated at 600° C. for        100 hours.    -   Cycle heating: “Heating in a nitrogen atmosphere having a dew        point of 80° C., at 600° C. for 30 minutes (in which the soaking        time is 25 minutes)→cooling in air for 5 minutes” is one cycle;        and the test piece is exposed to 500 cycles.

After tested in each heating mode, the test piece was visually checkedfor the presence or absence of red-brown coloration therein. Inaddition, the oxidation weight increase and the oxidation weightreduction (scale peel loss) were measured. All the test pieces of n=3were tested in the manner as above, and those having cleared the threerequirements of (i) presence of no red-brown coloration, (ii) oxidationweight increase of less than 0.2 mg/cm², and (iii) oxidation weightreduction of less than 0.2 mg/cm² were evaluated as good (O) and theothers were as not good (x).

[Peeling Resistance of Plating Layer]

A test piece of 25 mm×35 mm was cut out of the sample sheet; and thiswas tested in a mode of 600° C. cycle heating and 700° C. cycle heatingas mentioned below. In every test mode, the number of the test pieceswas n=5.

-   -   600° C. cycle heating: “Heating in a nitrogen atmosphere having        a dew point of 80° C., at 600° C. for 30 minutes (in which the        soaking time is 25 minutes) cooling in air for 5 minutes” is one        cycle; and the test piece is exposed to 1000 cycles.    -   700° C. cycle heating: “Heating in a nitrogen atmosphere having        a dew point of 80° C., at 700° C. for 30 minutes (in which the        soaking time is 25 minutes)→cooling in air for 5 minutes” is one        cycle; and the test piece is exposed to 1000 cycles.

After tested at the predetermined temperature, the test piece wasvisually checked for the presence or absence of peeling of the platinglayer. Of all the test pieces of 5 at different temperatures(n=5)×two-level temperatures, totaling 10, tested in the manner asabove, those with no peeling of the plating layer were evaluated as good(O) and the others were as not good (x).

The test pieces evaluated as good (O) in the above-mentioned 1000-cycletests were further tested up to 2000 cycles of 600° C. cycle heating and700° C. cycle heating. Of all the test pieces of 5 at differenttemperatures (n=5)×two-level temperatures, totaling 10, tested in themanner as above, those with no peeling of the plating layer wereevaluated as very good (Θ).

[Shapability]

A tensile test piece (JIS 13B) was cut out of the sample sheet (hot-dipAl-plated steel sheet having a thickness of 1.2 mm) in such a mannerthat its lengthwise direction could be the rolling direction of thesheet; and the test pieces of n 3 were tested for elongation in atensile test according to JIS 22241. The data of the elongation of thetest pieces of n=3 were averaged to give the mean elongation of thesample sheet. From various experiments, it is known that, when a steelsheet having a thickness of 1.2 mm could have an elongation of at least30%, then it may satisfy the necessary shapability in forming it intomotorcycle exhaust gas passageway members. Accordingly, the samplesheets having an elongation of at least 30% were evaluated as good (0)and the others were as not good (x).

[Low-Temperature Toughness]

A test piece of 55 mm×10 mm was cut out of the sample sheet (hot-dipAl-plated steel sheet having a thickness of 1.2 mm) in such a mannerthat its lengthwise direction could be perpendicular to the rollingdirection of the sheet, and its center was notched to have a 2-mmV-notch, thereby preparing a notched impact test piece. According to thedefinition by JIS 22202, the height is 10 mm, the width is 1.2 mm, thelength is 55 mm and the height below the notch is 8 mm. The test pieceswere tested in a Charpy impact test according to JIS 22242; and thosedetermined to have a nil ductility temperature (a brittle fractureoccurrence temperature) of not higher than −50° C. were evaluated asgood (O), and the others were as not good (x)

These results are shown in Table 2 and Table 3.

TABLE 2 Mean Plating Bath Thickness Red Scale Plating Composition ofPlating Resistance Layer Low- Sample Steel (% by mass) Layer continuouscycle Peeling Temperature Classification No. Code Al Si Others (μm)heating heating Resistance Shapability Toughness Example of 1 X1 balance9.0 — 4 ◯ ◯ Θ ◯ ◯ the Invention 2 X1 balance 4.5 — 20 ◯ ◯ ◯ ◯ ◯ 3 X1balance 4.5 — 14 ◯ ◯ Θ ◯ ◯ 4 X1 balance 11.5 — 13 ◯ ◯ Θ ◯ ◯ 5 X1 balance8.8 Ti: 0.15 18 ◯ ◯ ◯ ◯ ◯ 6 X1 balance 8.8 Ti: 0.15 11 ◯ ◯ Θ ◯ ◯ 7 X1balance 9.0 Sr: 0.40 8 ◯ ◯ Θ ◯ ◯ 8 X1 balance 8.7 B: 0.25 14 ◯ ◯ Θ ◯ ◯ 9X1 balance 8.5 10 ◯ ◯ Θ ◯ ◯ 10 X1 balance 8.2 B: 0.15 13 ◯ ◯ Θ ◯ ◯ 11 X2balance 9.0 — 6 ◯ ◯ Θ ◯ ◯ 12 X2 balance 3.9 — 14 ◯ ◯ Θ ◯ ◯ 13 X2 balance10.8 — 19 ◯ ◯ ◯ ◯ ◯ 14 X2 balance 8.9 Ti: 0.13 9 ◯ ◯ Θ ◯ ◯ 15 X2 balance9.0 Sr: 0.50 5 ◯ ◯ Θ ◯ ◯ 16 X2 balance 8.2 Cr: 0.13 10 ◯ ◯ Θ ◯ ◯ 17 X2balance 8.2 Mn: 0.20 13 ◯ ◯ Θ ◯ ◯ 18 X3 balance 9.0 — 18 ◯ ◯ ◯ ◯ ◯ 19 X3balance 8.7 Mg: 0.15 9 ◯ ◯ Θ ◯ ◯ 20 X4 balance 8.5 Zr: 0.14 14 ◯ ◯ Θ ◯ ◯21 X4 balance 9.0 — 18 ◯ ◯ ◯ ◯ ◯ 22 X4 balance 9.0 — 10 ◯ ◯ Θ ◯ ◯ 23 X5balance 9.0 — 11 ◯ ◯ Θ ◯ ◯ 24 X6 balance 9.0 — 8 ◯ ◯ Θ ◯ ◯ 25 X7 balance9.0 — 19 ◯ ◯ ◯ ◯ ◯ 26 X7 balance 9.0 — 9 ◯ ◯ Θ ◯ ◯ 27 X8 balance 9.0 —14 ◯ ◯ Θ ◯ ◯ 28 X9 balance 9.0 — 20 ◯ ◯ ◯ ◯ ◯ 29 X9 balance 9.0 — 10 ◯ ◯Θ ◯ ◯ 30 X10 balance 9.0 — 8 ◯ ◯ Θ ◯ ◯ 31 X11 balance 9.0 — 13 ◯ ◯ Θ ◯ ◯32 X12 balance 9.0 — 10 ◯ ◯ Θ ◯ ◯ 33 X13 balance 9.0 — 7 ◯ ◯ Θ ◯ ◯ 34X14 balance 9.0 — 17 ◯ ◯ ◯ ◯ ◯ 35 X14 balance 9.0 — 8 ◯ ◯ Θ ◯ ◯ 36 X15balance 9.0 — 13 ◯ ◯ Θ ◯ ◯

TABLE 3 Plating Bath Mean Red Scale Plating Composition ThicknessResistance Layer Low- Sample Steel (% by mass) of Plating continuouscycle Peeling Temperature Classification No. Code Al Si Others Layer(μm) heating heating Resistance Shapability Toughness Comparative 51 X1balance 9.0 — 42 ◯ ◯ X ◯ ◯ Example 52 X1 balance 9.0 —  2 X X Θ ◯ ◯ 53X1 balance 9.0 — 23 ◯ ◯ X ◯ ◯ 54 X2 balance 9.0 —  1 X X Θ ◯ ◯ 55 X2balance 9.0 — 60 ◯ ◯ X ◯ ◯ 56 X2 balance 9.0 — 25 ◯ ◯ X ◯ ◯ 57 Y1balance 9.0 — 15 ◯ ◯ X ◯ X 58 Y2 balance 9.0 — 18 ◯ ◯ ◯ X X 59 Y3balance 9.0 — 20 ◯ ◯ ◯ X X 60 Y4 balance 9.0 —  9 ◯ ◯ Θ X X 61 Y5balance 9.0 — 16 X X ◯ ◯ ◯ 62 Y6 balance 9.0 — 19 ◯ ◯ ◯ X X Underlined:falling outside the scope of the invention.

As known from Table 2, the examples of the invention in which the meanthickness of the Al-base plating layer falls within a range of from 3 to20 μm were all excellent in the red scale resistance and the platinglayer peeling resistance. In particular, those in which the meanthickness of the plating layer was less than 15 μm exhibited moreexcellent peeling resistance.

As opposed to these, Nos. 52 and 54 of comparative examples were poor inthe red scale resistance since the mean thickness of the Al-base platinglayer therein was too small. Nos. 51, 53, 55 and 56 were poor in theplating layer peeling resistance since the mean thickness of the Al-baseplating layer was larger than 20 μm. No. 57 was poor in the peelingresistance of the plating layer since the Nb content of the substratesteel sheet was too high. Nos. 58, 59, 60 and 62 were all poor in theshapability and the low-temperature toughness since the content of Cr,Ti, Mo and Cu in the substrate steel sheet was too high. No. 61 was poorin the red scale resistance since the Cr content of the substrate steelsheet was too low.

1. An Al-plated steel sheet for motorcycle exhaust gas passagewaymembers excellent in red scale resistance, which is produced by dippinga substrate steel sheet comprising, in terms of % by mass, at most 0.02%of C, at most 2% of Si, at most 2% of Mn, from 5 to 25% of Cr, from morethan 0.1 to 1% of Nb, at most 0.3% of Ti and at most 0.02% of N with abalance of Fe and inevitable impurities, in a hot-dip plating bathcontaining, in terms of % by mass, from 3 to 12% of Si with a balance ofAl and inevitable impurities, then pulling it up, and controlling theplating amount to thereby form a plating layer having a mean thicknessof from 3 to 20 μm on the surface thereof.
 2. The Al-plated steel sheetfor motorcycle exhaust gas passageway members excellent in red scaleresistance as claimed in claim 1, wherein the steel sheet of thesubstrate further contains at least one of at most 0.6% of Ni, at most0.2% of Al, at most 3% of Mo, at most 3% of Cu, at most 3% of W, at most0.5% of V, at most 0.5% of Co and at most 0.01% of B.
 3. The Al-platedsteel sheet for motorcycle exhaust gas passageway members excellent inred scale resistance as claimed in claim 1, wherein the hot-dip platingbath further contains at least one of Ti, B, Sr, Cr, Mn, Mg and Zr in atotal amount of at most 1%.
 4. A motorcycle exhaust gas passagewaymember which is formed of the plated steel sheet of claim 1 as theconstitutive material thereof and which is so designed that the platinglayer thereof is kept in contact with exhaust gas and that the maximumservice temperature is 400° C. or higher.
 5. The Al-plated steel sheetfor motorcycle exhaust gas passageway members excellent in red scaleresistance as claimed in claim 2, wherein the hot-dip plating bathfurther contains at least one of Ti, B, Sr, Cr, Mn, Mg and Zr in a totalamount of at most 1%.
 6. A motorcycle exhaust gas passageway memberwhich is formed of the plated steel sheet of claim 2 as the constitutivematerial thereof and which is so designed that the plating layer thereofis kept in contact with exhaust gas and that the maximum servicetemperature is 400° C. or higher.
 7. A motorcycle exhaust gas passagewaymember which is formed of the plated steel sheet of claim 3 as theconstitutive material thereof and which is so designed that the platinglayer thereof is kept in contact with exhaust gas and that the maximumservice temperature is 400° C. or higher.